There is a well known fastener driving tool having a housing in which pressurized air is supplied and a cylinder is located. A piston is reciprocatingly received within the cylinder. The piston has a driving rod as a driver to drive a fastener onto an object. The pressurized air in the housing is fed to the upper portion of the cylinder, so that the piston abruptly moves downwardly. Thus, the driver drives a fastener placed in a nose of the tool onto the object. A return-air-chamber is formed between the outer surface of the cylinder and the inner wall of the housing to store compressed air therein. Then, the pressurized air is exhausted from the upper portion of the cylinder to the atmosphere. The compressed air in the return-air-chamber is fed to the lower surface of the piston to return the piston to the upper position of the cylinder. A bumper is disposed on the lower end of the cylinder to absorb shock or impact caused by the abrupt down-movement of the piston. Such a fastener driving tool is described in, for example, Japanese U.M. Publication No. 4-53908.
In the driving tool described in the above publication, an improved return-air-chamber is formed to overcome a problem that the piston sometimes fails to return to the upper dead point if the capacity of the return-air-chamber is small. In the driving tool of the prior art, an additional air chamber 9 is formed below the bumper 11 to receive the pressurized air from the return-air-chamber 7 to increase the capacity of the return-air-chamber. A cavity 13 is formed in the lower potion of the bumper 11 to collect pressurized air from the additional air chamber 9 to increase a shock absorbing force of the bumper and to reduce heat generation in the bumper as well as prevention of improper return of the piston. Nevertheless, this known driving tool has another problem to be overcome. In a driving operation, the piston strikes and compresses the bumper. The compressed bumper reacts to press the piston upwardly. On the other hand, high air pressure is continued to be applied to the upper surface of the piston. Thus, both the piston and the driver are forced to again be pressed down. That is, so-called double-driving or twice-driving occurs. The double-driving is undesirable. At the first driving stage, the driver properly strikes a fastener such as a nail to be driven into an object. At the second driving stage in the double-driving, however, the driver hits not only the fastener but the object such as a decorative panel to damage the hit area of the object.